1. Field of the Invention
The invention relates to a power amplifier for amplifying an analog input signal.
2. Background Information
Such a power amplifier is disclosed, for example, in DE-A1-3,502,135. the power amplifier disclosed therein is composed of a plurality of identical individual amplifiers which are connected in series at their outputs and are each composed of a switching amplifier connected to the secondary and a subsequently connected lowpass filter.
According to FIG. 1, the switching amplifiers connected to the secondaries are each composed of a direct voltage mains unit UB1 and UB2, respectively, and a subsequently connected switch SCH1 and SCH2, respectively, which is actuated by a pulse train derived from the analog input signal to be amplified. In order to bridge the respective direct voltage mains unit UB1 and UB2, when the respective switch SCH1, SCH2 is open, a free running diode FD1, FD2, respectively, is provided at its output.
Such a direct voltage unit (e.g. UB1 in FIG. 1) is customarily composed of a mains transformer configured, for example, as a three-phase transformer having three delta connected primary windings and three wye connected secondary windings. Customarily the secondary windings are connected to a three phase bridge rectifier which, in order to smooth the resulting raw direct voltage, is followed by a filter member including a filter choke and a filter capacitor. The filter choke is connected between the positive pole of the rectifier circuit and the output terminal having the higher potential, while the filter capacitor is connected in parallel with the two output terminals.
According to the present state of the art, a mains voltage of 380 V at a frequency of 50 Hz or 60 Hz can be employed at the primary side of the mains transformer for a direct voltage mains unit up to a power of approximately 200 kW at a direct voltage of approximately 30 kV. For powers above 250 kW, the primary currents occurring at a 380 V mains voltage become very high and thus difficult to manage from an engineering point of view. In this case, voltages between 4 kV.sub.eff and 30 kV.sub.eff are employed on the primary side of the mains transformer.
A very economically producible mains transformer is configured as a so-called cast resin transformer which makes it possible to bring the many terminals of the secondary windings out in an economical manner. However, such a cast resin transformer can be employed economically only for a primary voltage of 380 V.
FIG. 2 is a cross-sectional view of such a cast resin transformer TR which is configured as a three-phase transformer. Three coil units r, s, t corresponding to the three phases of the three-phase current are wound around an iron core 1 configured, for example, to have a so-called EI-type lamination. Each coil unit is composed of a primary coil 2 wound directly around the iron core 1, a grounded shielding 3 surrounding the primary coil, and a secondary winding 4 whose windings are encased in cast resin surrounding the shield. For electrical insulation, an air channel 5 having, for example, a dielectric strength of 50 kV is provided between shielding 3 and secondary winding 4. Between iron core 1 and primary winding 2, between primary winding 2 and shielding 3, between shielding 3 and secondary winding 4 there exists in each case a dielectric strength of at least 2.5 kV. FIG. 2 shows that air channel 5 which, in a disadvantageous manner, takes up a large amount of space, is provided only once. Electrical insulations for insulating voltages of 2.5 kV, however, can be provided in a much more space saving manner, for example with the aid of plastic sheets. The terminals can then be easily brought out of the cast resin on the exterior face of the cast resin coil.
If now a mains transformer were constructed according to the same technology for powers greater than 250 kW, that is for a mains input voltage of, for example 20 kV, the insulating air channel 5 for 50 kV would have to be provided three times, namely between the iron core 1 and primary winding 2, between primary winding 2 and shielding 3 and between shielding 3 and secondary winding 4. Such a mains transformer would have very large dimensions and could therefore not be employed economically.
For such high power applications (greater than 250 kW) it is therefore possible only to employ a mains transformer in which a liquid insulating material, for example a so-called transformer oil having the highest possible dielectric strength, is employed. Although this permits the construction of a spatially relatively small mains transformer since the electrically insulated distances can be selected to correspond to the minimum dielectric strength of the liquid insulating material, the dielectric strength with respect to air is considerably higher and is, for example, at least 30 kV/2.5 mm.
One embodiment of such a mains transformer is shown in a sectional view in FIG. 3. The longitudinal sectional view shows one leg r of a three-phase transformer TR filled with liquid insulating material 5' (e.g. transformer oil) and comprising an iron core 1, a primary winding 2, a grounded shielding 3 as well as several secondary windings 4.sub.1 to 4.sub.3 arranged in the chamber winding technique. Due to the rotational symmetry of the structure of leg r, only one half of the leg r is illustrated. The axis of symmetry SA here extends through iron core 1.
The lowpass filters in the prior art power amplifier are each composed of an LC half member including a filter coil FL1 and FL2, respectively, and a filter capacitor FC1 and FC2, respectively. The upper cut-off frequency of the lowpass filters is dimensioned so that the analog input signal appears amplified at the amplifier output and is transmitted to the load R essentially without distortion.
In the prior art power amplifier, the series connection of the individual amplifiers is realized in that the filter capacitors FC1 and FC2, respectively, of the individual LC half members are connected in series.
Similar power amplifiers, but equipped with switching amplifiers at their primaries and with connected individual amplifiers are disclosed in EP-A1-0,025,234 (U.S. Pat. No. 4,369,409) and DE-A1-2,841,833 (U.S. Pat. No. 4,164,714).
In order to actuate the switching elements in the individual switching amplifiers, the prior art power amplifiers employ pulse duration modulated ("PDM") pulse trains which are derived in known circuits by high frequency sampling of the analog input signal and in which the width and the duration of the individual pulses is directly proportional to the sampled momentary value of the amplitude of the analog input signal. Insofar as the sampling or switching frequency (which is constant for PDM) is at least twice as high as the maximum permissible frequency of the input signal, each individual one of these pulse trains contains the complete information about the analog input signal to be amplified, with special embodiments of such power amplifiers providing for the analog input signal to be sampled by the pulse trains with a shift in phase relative to one another. The cut-off frequency of the individual lowpass filters is therefore selected so that the sampling frequency including its harmonics are filtered out of the pulse duration modulated power pulses at the output of the respective switching amplifier, and in this way, the analog input signal is amplified and is put out essentially without distortion at the output of the lowpass filter.
The dimensioning of the individual lowpass filters in the prior art power amplifiers is based solely and uniquely on the capacitances and inductances of the filter capacitors (FC1 and FC2 in FIG. 1) and filter coils (FL1 and FL2 in FIG. 1) employed in the lowpass filters.
While in the prior art power amplifiers with correspondingly dimensioned lowpass filters, an almost distortion-free amplification of the analog input signal can definitely be realized, the high efficiency theoretically to be expected due to the use of switching amplifiers instead of linear amplifiers is in no way realized in practice with the prior art amplifiers.